The present invention relates to variable transmission or reflection devices, and more particularly, to rearview mirrors capable of automatically changing reflective states in response to control signals derived from varying light levels or manual controls. Moreover, this invention is directed to such mirrors having areas of constant light transmission.
High light levels reflected from a rearview mirror into the eyes of a vehicle operator during night driving can seriously impair the operator's forward vision. Light scatters within the eye causing a "veil" or "curtain" of light over the scene. The driver, therefore, has a reduced ability to detect objects which are dimly lit or have little contrast with the background. This situation is known as "disability glare."
In an effort to alleviate the problems of undesired glare at night, rearview mirrors have recently been equipped with electro-optic devices which reduce the transmission of reflected light with light scattering liquid crystal layers or coloration via electrochromic layers. The light scattering or coloration is responsive to light entering the rear of the vehicle such as from following car headlights and incident upon a photo sensor which controls the current to the electro-optic device.
In order to use open-loop glare detection circuitry of the type disclosed in U.S. Pat. Nos. 4,793,690 to Gahan et al., 4,799,768 to Gahan and 4,886,960 to Molyneux et al., each of which require the photo sensor to have a clear, unobstructed view of the light conditions to the rear of the vehicle, one type of electro-optic mirror includes a sensor in the bezel or housing for the mirror assembly adjacent the reflective surface. However, in order to contain the sensor, the bezel or housing must have a depending portion or chin substantially broader than the remaining portion of the bezel. To some, such housings are less desirable aesthetically. In addition, such mountings require additional mechanical design and tooling and restrict mirror case design options.
Other electro-optic mirrors have included the sensor behind the reflective element of the mirror assembly. A portion of the reflective layer is removed to allow light directed from the rear of the vehicle to pass through the glass layers to the sensor. The sensor then controls a closed-loop detection circuit which generates a signal which is used to darken the electro-optic medium in front of the reflective glass panel. A major disadvantage with locating the sensor behind the reflective surface in electro-optic devices is that when the electro-optic device darkens, the amount of light incident upon the sensor is reduced which, in turn, causes the electro-optic device to lighten. As the coloration decreases, the light entering through the rear of the vehicle is again incident on the sensor which again causes the electro-optic device to darken. Thus, the sensor, indirectly controlled by the electro-optic device, is continually wavering between an on and off position. Additional electronic circuitry is then required to compensate for such instability, and glare control is often less than optimum.
Accordingly, a need has existed for an electro-optic mirror which allows the mounting of a photo sensor within the reflective surface of the mirror but avoids alteration of the sensed light due to mirror dimming while allowing use of precisely controlled open-loop detection circuits. Additionally, a need has existed for an electro-optic mirror allowing use of information displays in the reflective viewing surface whose intensity is unaffected by mirror dimming operation.